Impelled by environmental concerns and increasing governmental regulations on the volatile organic content of coatings, the applications of water-based flexographic and rotogravure inks are increasing in the ink industry. A typical water-based ink system is formulated by the addition of a binder resin (usually acrylic polymer latices) to a grind base. Grind bases are prepared by using grinding resins to disperse pigments. Pigments are crystalline solids composed of agglomerates, aggregates, and primary particles which vary in size from 0.02 to 0.5 microns. During the grinding process, agglomerates and aggregates are broken down into primary particles which possess strong tendencies to reassociate in an ink. The finer the particle size of the pigment, the greater the color strength, but the more difficult the pigment becomes to disperse. Grinding resins help prevent the particles from reassociating by increasing both the electrostatic and steric repulsion between pigment particles.
An ink formulator must consider the compatibility of the various ink components when selecting the grind resins to be used. Grind bases (i.e., pigment dispersions) are let down with a variety of alkali-soluble resins or alkali-insoluble resin emulsions to achieve the properties desired for the end use of the ink. If the grind resins and the let-down resins are not compatible, the result may be pigment flocculation, viscosity increase, loss of color strength, and other problems.
Currently, the rosin resins UNIREZ.RTM. 7098 (manufactured by Union Camp Corporation) and FILTREZ.RTM. 5014 (manufactured by Filtered Rosin Products Corporation) are pre-eminent in the alkali-soluble grind resin market. These resins are unique in their ability to produce stable, low viscosity, high solids grind bases and inks for water-based flexographic printing. (As used herein the term "high solids" means containing over 20% solids.) UNIREZ 7098 and FILTREZ 5014 have high softening points (130-140.degree. C.), produce excellent color development when grinding pigment, and contribute to the toughness and water resistance of the finished ink. However, both the viscosity and the stability of grind bases and inks produced from UNIREZ 7098 or FILTREZ 5014 are highly pH dependent. As grind bases must remain stable and fluid to be applicable, this pH dependence has remained a problem associated with the use of these resins.
At the present time, most water-based printing inks are produced at pH values between 8 and 10. It would be advantageous to ink formulators to have a resin which is stable over this entire range. Therefore, it is the object of this invention to have an alkali-soluble resin which may be used to prepare stable, low viscosity pigment dispersions over a wide range of pH values while maintaining excellent physical properties in the finished inks.